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Information from Your Family Doctor

Prenatal Diagnosis: Amniocentesis and CVS

Am Fam Physician. 2005 Oct 15;72(8):1577-1578.

What is prenatal diagnosis?

Prenatal diagnosis is a way for your doctor to tell if your baby has a problem such as Down syndrome while you’re still pregnant. Amniocentesis (say: AM-nee-oh-sen-TEE-sis) and chorionic villus sampling (say: CORE-ree-on-ik VILL-us SAM-pling, or CVS, for short) are tests that help find problems before your baby is born. Knowing about these problems beforehand can help you decide how to care for your baby. Some problems can be treated before your baby is born. Other times your baby might need special treatment right after he or she is born.

Do all pregnant women have these tests?

No. The tests can be useful if you are 35 years or older when your baby is due. Women older than 35 years have a greater risk of having a baby with a gene problem such as Down syndrome. You should think about having these tests if you have had a child with Down syndrome or another problem, such as spina bifida (say: SPY-nuh BIFF-uh-duh). Parents who have family members with a gene disorder, such as cystic fibrosis (sa: SIS-tik FY-bro-siss), also might want to have these tests.

What is amniocentesis?

Amniocentesis is when your doctor uses a thin needle to take some of the fluid out of your womb for tests. Your body will make more fluid to replace what is taken out. The baby will not be hurt. Some women feel mild cramping during or after amniocentesis. Your doctor may tell you to rest on the day of the test, but you usually can get back to your normal activities the next day.

What is CVS?

CVS is when your doctor takes a small piece of the placenta from your womb. Your doctor will give you medicine to keep you from feeling any pain. Your doctor will use a thin tube or needle to take the sample. Most women feel fine after the test, but some may have mild bleeding.

When are the tests done?

Amniocentesis usually is done in the 15th week of pregnancy or later. CVS usually is done between the 10th and 12th weeks of pregnancy.

Is one test better than the other?

Amniocentesis is better than CVS for some women. You should have amniocentesis if you have had a baby with a neural tube defect, such as spina bifida, or if you or your partner has a neural tube defect. CVS does not test for these problems.

Amniocentesis may be better if the results of other tests have not been normal. CVS may be better if you and your doctor want to know the test results during your first three months of pregnancy.

Are there risks involved with these tests?

Both tests have a small risk of miscarriage. In some babies, CVS may cause problems with fingers or toes. This only seems to happen if the test is done before the ninth week of pregnancy. Your doctor will talk with you about the risks of amniocentesis and CVS.